The use of chlorine dioxide as a bleaching agent and disinfectant is well known. In particular, the use of chlorine dioxide as a disinfectant in both industrial and potable water systems has become increasingly important in recent years because in contrast to chlorine, the most widely used oxidising biocide, its use does not give rise to the significant production of trihalomethanes. However the adoption of chlorine dioxide has been restricted because of the hazardous nature of the chemical.
Chlorine dioxide is an unstable gas which is explosive at pressures greater than 40 kPa. (3000 mnHg). It has been found impossible to compress and store chlorine dioxide gas either alone or in combination with other gases. Chlorine dioxide is therefore manufactured at its point of use. The equipment used to produce chlorine dioxide is costly and has to take account of the hazardous nature of the chemical. Large consumers of the chemical, e.g. those involved in the bleaching of wood pulp, have used somewhat; complicated processes based on the reduction of sodium chlorate. For use in smaller applications oxidation of chlorite is favoured. However all these processes require considerable capital expenditure, an understanding of the chemistry involved and skilled personnel to operate the units efficiently and safely.
There is therefore a need to be able to produce chlorine dioxide safely and cost effectively in relatively small quantities that will allow a greater number of industrial and potable water systems to take advantage of the superior disinfection and stability properties of the chemical without the need for large capital investments and specially trained personnel.
To an extent this need has been satisfied by the introduction in recent years of "stabilised" solutions of chlorine dioxide sold under a variety of trade names. These products claim to be solutions of chlorine dioxide stabilized in solution through the formation of a variety of complexes.
Thus for example, the producers of Purogene claim to have produced a stable aqueous solution whose active ingredient is chlorine dioxide. They state that during water treatment 50-70% of the chlorine dioxide reacted will immediately appear as chlorite and the remainder as chloride. The chlorite, it is stated, will continue to react with remaining oxidisable material reducing entirely to chloride. The reactions occurring being as follows:
(1) Cl0.sub.2 +e.sup.-.fwdarw.Cl0.sub.2.sup.- (chlorite) PA1 (2) Cl0.sub.2.sup.- +4H.sup.+ +4e-.fwdarw.Cl.sup.- +2H.sub.2 0 (chloride) PA1 a chlorite, PA1 a chlorine donor, PA1 an alkali, and PA1 water, PA1 the chlorite and chlorine donor being present in a molar ratio of from 1.0:0.1 to 1.0:15.0 chlorite to chlorine donor, PA1 the alkali being present in an amount sufficient to ensure a pH of above 11 and the water being present in an amount to give a theoretical minimum concentration of 0.5 ppm chlorine dioxide. PA1 quaternary ammonium and phosphonium compounds amines, iso-thiazolone mixtures and thiocyanates; PA1 and chemicals which are known to provide cleaning and penetration when combined with biocides such as surfactants particularly non-ionic surfactants.
Viscona limited claim to have a 5% (50,000 ppm) aqueous stabilised chlorine dioxide solution chemically buffered at a pH of 9 which releases chlorine dioxide in around 20 minutes when activated. Release of chlorine dioxide is achieved by lowering the pH of the solution to approximately 2 using a suitable acid (with a chlorine donor for rapid results). Activation with citric acid converts only approximately 10% of the available chlorine dioxide to free chlorine dioxide, in aqueous solution, after about 15 minutes. It is stated subsequent activation would continue at a very slow rate. Such a method is not sufficiently rapid for use in disinfection where a need for an activation rate of 50% or more is required.
The rate of activation can be increased using a stronger acid. For example adding 30 to 35% hydrochloric acid to bring the pH down to 1.5 activates 15% of the potential chlorine dioxide in 1 hour, 25% in 2 hours and 50% in 24 hours.
By adding a chlorine donor, e.g. hypochlorite, around a 70 to 80% release in about 15 minutes can be achieved.
Another product, OCS Dioxide produced by Odour Control Systems Limited, is stated to be a combination of oxygen and chlorine joined as chlorine dioxide in aqueous solution.
Chlorine dioxide is generated from these solutions by reacting them with acids, particularly strong acids if a significant release of chlorine dioxide is required in a reasonable period of time. A common approach with these products is to dilute the product in a mixing tank with water to give a solution which contains a theoretical concentration of about 2-3000 ppm chlorine dioxide and then add sufficient strong acid, hydrochloric acid or phosphoric acid most commonly, to reduce the pH to within the specified pH range. The chlorine dioxide is then released from the complex into solution over a period of time which can vary from a few minutes to many hours depending primarily on the pH and the strength of the solutions. The solution is then proportionately dosed to the system to provide the required reserve of chlorine dioxide. The "stabilised" chlorine dioxide is never fully released from the complex and conversion rates to "free" chlorine dioxide are quoted as varying from 15% to 75% depending upon pH, concentrations and time.
It is clear that while the introduction of these "stabilised" solutions has provided a means of utilising chlorine dioxide without the need for complex and costly capital equipment they have not fully addressed many problems associated with utilising chlorine dioxide safely and effectively. In particular strong acids have to be used to produce disinfecting amounts of chlorine dioxide, the concentrations and reaction times of the various ingredients have to be carefully controlled to maximise the production of chlorine dioxide and finally the solution has to be dosed proportionately to the system to achieve the biocidal concentration of chlorine dioxide.
In addition the preparation of these solutions is expensive as the chlorine dioxide has to be first generated, dissolved into water and then finally stabilised.